This invention relates to ozone resistant co-cured blends of ethylene propylene diene monomer elastomers (EPDM) dispersed in natural rubber (NR). More specifically, it relates to compositions of the type described which are particularly well adapted to being cured in the presence of oxygen, such as is encountered when curing in an autoclave, and at relatively low temperatures (e.g. 300.degree. F.).
There are many product applications for rubber that require the products to undergo curing in a pressurized air environment such as is typically employed in autoclave curing. Many such products comprise relatively thin layers of rubber material which can also be layered onto other materials, such as fabric or other structural materials that are sensitive to high temperatures, e.g. above 300.degree. F. Typical examples of such products would be rubber protective footwear, waders protective clothing and gloves. Examples of other rubber products (with or without associated temperature sensitive materials) that normally use pressurized air cure would be rubber rollers (especially large articles), large rubber hoses and tire sidewalls.
Natural rubber is particularly desirable for use in products of the type just described because of its inherent tack and good adhesion property in the uncured state, as well as its processability, and also because of its excellent tensile properties and favorable economics. However, natural rubber is known to lack chemical resistance and, in particular, is susceptible to ozone degradation caused by the high unsaturation level in its molecular structure. Ozone generally attacks carbon-carbon double bonds to form ozonide that creates surface cracking. Upon stress, these microscopic cracks become macroscopic which causes undesirable leakage in footwear.
It is known to use additives dispersed in the natural rubber to improve ozone resistance. Wax has been employed to form a protective layer that insulates the rubber surface from ozone attack. However, due to the brittle nature of wax this solution has not been found to be effective when the rubber product is subjected to cyclic stress in use. Alternatively, chemical antiozonants have been developed that bloom to the surface of the rubber and scavenge ozone. These chemical antiozonants increase the critical energy level for crack initiation and thus enhance the ozone resistance of the natural rubber. Unfortunately, the bloom of antiozonants to the rubber surface also introduces adhesion problems that are highly undesirable when products are produced by plying layers of rubber together as is the case with rubber footwear and similar products.
A more recent approach to improving ozone resistance involves the practice of blending natural rubber with ethylene propylene rubber (EPR) to take advantage of the fact that the dispersed EPR particles reduce crack length and increase the critical value of energy for macroscopic cracking. The terpolymer of EPR, ethylene propylene diene monomer (EPDM) rubber, has small pendant unsaturation sites such as ethylene norbornene (ENB), dicyclopentadiene (DCPD), etc., which make the EPDM sulfur curable and allow it to be blended with highly unsaturated elastomers such NR, SBR and NBR. The degree of dispersion of EPDM particles and the molecular weight of EPDM influence the blend's ozone resistance and tensile properties. Blends of 30%-35% EPDM with 70%-65% NR are known to be used to improve ozone resistance. Unfortunately, however, blends of these constituents are also known to suffer from lack of tack (adhesion) property and low tensile strength. Both of these problems are attributed to the low polarity of EPDM and its cure incompatibility with natural rubber. It is known that building tack and processability of NR/EPDM blends can be improved by the addition of small amounts of low molecular weight (liquid-like) EPDM. However, the difficulty in achieving a satisfactory co-cure of the NR and EPDM in the blend still contributes in a major way to resultant blends that exhibit poor tensile strength and poor ozone resistance.
The use of sulfur as a rubber curing (vulcanizing) agent is well known. However, the use of sulfur alone as the curing agent in NR/EPDM blends has been found to be less than fully satisfactory because of the demonstrated preference of sulfur to the NR phase in the curing process which has the effect of leaving the EPDM phase relatively uncured. Surprisingly, however, it has been found that the addition of an organic peroxide as a second curing agent in combination with sulfur improves the co-curability of the NR and EPDM and permits the development of a co-cured NR/EPDM blend with improved ozone resistance while at the same time achieving the desired tensile properties.
Organic peroxide is known to cure equally well with either NR or EPDM. The mechanism of organic peroxide curing is based on the fact that organic peroxide molecules, in general, have thermally unstable oxygen-oxygen bonds in common that, when heated, break and form free radicals that can extract hydrogen atoms from polymer chains. The resultant polymer radicals easily combine to form stable carbon-carbon bonds that greatly exceed crosslink bonds produced by sulfur curing. Unfortunately, however, heating of organic peroxide to produce the free radicals useful for crosslinking purposes is well known to make the peroxide highly susceptible to rapid degradation when exposed to oxygen. As a consequence, it has not generally been considered feasible to use organic peroxide in an air cure system such as is the case with autoclave curing generally used in many non-tire product applications of the type described above. Quite surprisingly, however, it has been discovered, in accordance with a feature of the present invention, that the joint use of sulfur and organic peroxide as co-curing agents in NR/EPDM blends results in excellent co-cure of the NR and EPDM without degradation or decomposition of the organic peroxide even when used in an autoclave air curing type of system. Moreover, is has been found that co-cure of the blend is achieved at moderately low temperature, e.g. 300.degree. F., which makes it particularly attractive in the production of certain built-up rubber items, such as footwear, having structural underlayment materials in them that can be damaged when exposed to temperatures higher than 300.degree. F.
It is therefore an object of the present invention to provide a composition comprised of an NR and EPDM blend that has improved ozone resistance over known NR/EPDM blends. It is a further object of the invention to provide a co-cured NR/EPDM blend that has improved tensile strength over prior art NR/EPDM blends. It is yet a further object of the invention to provide an NR/EPDM blend that is co-curable in an air cure environment such as encountered in autoclave curing and at relatively low temperataures of, for example, 300.degree. F. It is a still further object of the invention to provide a method of producing co-cured NR/EPDM rubber blends in an air environment that exhibit improved ozone resistance and tensile strength properties over prior art air cured blends of this type.